With a rapid development of various information appliances from mobile terminals to large-capacity servers, even higher performance, such as higher integration, faster speed, and reduced power consumption, has been sought for components like memory and logic elements used to configure such appliances. In particular, a significant advance has been made in a non-volatile semiconductor memory, and a flash memory as a large-capacity file memory is increasingly widespread and is forced to take the place of hard disk drives.
Meanwhile, regarding expansion to code storage and even to a working memory, the development of non-volatile semiconductor memories is also advancing so as to replace an NOR flash memory, a DRAM, and the like that are currently commonly used. For example, Ferroelectric Random Access Memory (FeRAM), Magnetic Random Access Memory (MRAM), Phase Change RAM (PCRAM), and the like can be exemplified as the non-volatile semiconductor memories. Some of these have already been commercialized.
Among these non-volatile memories, MRAM stores data using a magnetization direction of a magnetic material, and is therefore capable of fast-speed rewriting and nearly infinite rewriting (1015 times or more). MRAM is already used in fields such as industrial automation or aviation.
Due to its fast-speed operation and reliability, there are high expectations for future use of MRAM as code storage and working memory.
However, reducing power consumption and increasing capacity remain as issues for MRAM.
These are important problems due to a storage principle of MRAM, that is, a method by which magnetization is reversed using a current magnetic field generated from the wiring.
As a method of solving this problem, storage methods (that is, magnetization reversal) that do not rely on a current magnetic field are being studied, and among them, research relating to spin torque magnetization reversal is actively ongoing (refer to, for example, PTL 1, PTL 2, PTL 3, NPL 1, and NPL 2).
Hereinafter, an MRAM which uses spin torque magnetization reversal is referred to as a Spin Torque Transfer based Magnetic Random Access Memory (STT-MRAM). Spin torque magnetization reversal is sometimes also referred to as spin injection magnetization reversal.
STT-MRAM comes in two types which are an in-plane magnetization type and a perpendicular magnetization type. Among them, in recent years, a perpendicular magnetization type of STT-MRAM which is more suitable for scaling is actively being developed.
For example, according to NPL 3, it has been suggested that using a perpendicular magnetization film such as a Co/Ni multilayer film in the storage layer makes it possible both to reduce the reversal current and to ensure thermal stability.